This invention relates to an apparatus for cleaning bag filters, particularly filters of the type utilized in industrial systems for dust collection wherein a system may comprise a large housing which may have enclosed therein a plurality of elongated tubular bags, and a ducting system conveying dust-laden air into the housing and clean air from the housing with the assistance of a blower system mounted somewhere in the ducting system.
Industrial filtering systems are used for removing particulate matter from air and other gaseous mixtures for recycling in industrial control processes, and also for cleaning the environmental air supplied to work stations in an industrial plant. In either event it is frequently necessary for such systems to handle tremendous volumes of air flow continuously, and in a manner in which the filtering components can be periodically cleaned so as to maintain the air flow efficiency over long periods of time. Without special filter cleaning mechanisms any filtering system will eventually accumulate such quantities of particulate matter so as to impede further flow of air therethrough, thereby reducing the delivery rate of air. Typically such cleaning systems are serially inserted into a ducting system so that all air passes through the filter apparatus and wherein the particulate matter is trapped within the apparatus and the clean air is permitted to pass through the apparatus. Since such a system is serially connected into a ducting network, means must be provided for removing the collected particulate matter from clogging the air flow passages. Large filter bag housings have been constructed to manage this problem, wherein a plurality of tubular bag members are coupled to a dirty air duct so as to force the dirty air downwardly or upwardly through the tubular bag interior, and a clean air duct is connected to the housing so as to be in air coupling arrangement with the bag exterior. A pressure differential is therefore developed across the bag surface wherein air passes through the surface while the bag material impedes the passage of the particulate matter which is to be filtered from the air flow. Bag materials are selected so as to have the desired density for the particle sizes encountered in any particular environment. As such a system operates particulate matter becomes accumulated along the inner bag surfaces, although some of the particulate matter breaks free from the bag and falls downwardly through the open bag tubular end into a collection hopper. However, a considerable amount of particulate matter does not fall free and must be physically removed by means of a mechanical or other type of bag shaking mechanism. In the prior art, mechanical shakers have physically vibrated the bag periodically in order to dislodge particulate matter so accumulated. Other prior art systems have utilized an air pressure reversal technique for periodically reversing the pressure drop across the bag surface, thereby collapsing the bags having accumulate particulate matter along their inner surface and physically and forcibly dislodging such matter in this manner. Such pressure reversal devices have required valving or gating mechanisms coupled to the delivery ducting system so as to shut off the inlet air supplied to the system and to disconnect the outlet air temporarily until a reverse pressure air supply can be injected into the bag housing.
Mechanical bag shaking systems have suffered from the obvious disadvantage of high cost and unreliability, for the driving mechanism must be of significant power capability. Typical bag filter housings in industrial applications may contain 12 - 30 filter bags, each bag having a diameter of about 8 - 12 inches and a length of from 12 - 20 feet. Therefore, a mechanical apparatus for physically shaking the bags in such a system requires a power source of significant magnitude. The cost of providing this power source, together with the mechanical difficulties and unreliability of the various mechanical couplings makes such a system undesirable in most industrial applications.
The air pressure reversal type of bag cleaning system is a more preferred approach to the problem, for it operates smoothly by delivering a reverse pressure blast into the housing which is of sufficient magnitude so as to collapse all of the bags in the housing. However, this approach requires a considerable volume of reverse air flow to be held in reverse for the cleaning blast to be effective, and also requires gating mechanisms for interrupting the normal flow paths through the ducting system. This system is somewhat destructive of bags, particularly bags which may have accumulated an excessive amount of particulate matter and are therefore unable to relieve the pressure stresses which are suddenly imposed upon them. The collapse of a bag which is nearly clogged with particulate matter may be so extreme as to rupture the bag and require replacement. To the extent that some bags in a filter housing having accumulated a greater or lesser layering of particulate matter they will be effective to a greater or lesser degree by the reverse air pressure and will be physically stressed to a greater or lesser degree. Thus, while some bags in the housing may be shaken very little others may collapse to the point of bag rupture and thereby disable the effectiveness of the entire system. These problems have caused users of the reverse pressure bag cleaning technique to place supporting grids internally of the bag to prevent bag collapse beyond a predetermined distance, and have required such systems to limit the maximum length of bag which may be used in any particular application. Such systems have also found it necessary to compartmentalize a series of shorter bag lengths into a plurality of zones for cleaning in order to control the maximum degree of bag collapse which may exist in any one zone. While all of these solutions have proved workable, they add to the cost and complexity of a filter cleaning system and are therefore undesirable.